Cardiac output, the volumetric rate at which blood is pumped through the heart, is most often determined clinically by injecting a bolus of chilled saline or glucose solution into the right auricle or right ventricle through a catheter. A thermistor disposed in the pulmonary artery is used to determine a temperature-time washout curve as the chilled injectate/blood mixture is pumped from the heart. The area under this curve provides an indication of cardiac output. Although this thermo-dilution method can give an indication of cardiac output at the time the procedure is performed, it cannot be used for continuously monitoring cardiac output. Moreover, the frequency with which the procedure is performed is limited by its adverse effects on a patient, including the dilution of the patient's blood that occurs each time the chilled fluid is injected. In addition, the procedure poses an infection hazard to medical staff from blood contact, and to the patient, from exposure to possibly contaminated injectate fluid or syringes.
Alternatively, blood in the heart can be chilled or heated in an injectateless method by a heat transfer process using a temperature-conditioned fluid that is pumped in a closed loop, toward the heart through one lumen within the catheter and back through another lumen. The principal advantages of using such a non-injectate heat transfer process to change the temperature of blood are that the blood is not diluted, and the temperature differential between the blood and the heat exchanger is much less compared to the temperature differential between an injectate fluid and blood in the typical thermal dilution procedure.
U.S. Pat. No. 4,819,655 (Webler) discloses an injectateless method and apparatus for determining cardiac output. In Webler's preferred embodiment, a saline solution is chilled by a refrigeration system or ice bath and introduced into a catheter that has been inserted through a patient's cardiovascular system into the heart. The catheter extends through the right auricle and right ventricle and its distal end is disposed just outside the heart in the pulmonary artery. A pump forces the chilled saline solution through a closed loop fluid path defined by two lumens in the catheter, so that heat transfer occurs between the solution and blood within the heart through the walls of the catheter. A thermistor disposed at the distal end of the catheter monitors the temperature of blood leaving the heart, both before the chilled fluid is circulated through the catheter to define a baseline temperature, and after the temperature change in the blood due to heat transfer with the chilled saline solution has stabilized. Temperature sensors are also provided to monitor both the temperature of the chilled saline solution at or near the point where it enters the catheter (outside the patient's body) and the temperature of the fluid returning from the heart. In addition, the rate at which the chilled solution flows through the catheter is either measured or controlled to maintain it at a constant value. Cardiac output (CO) is then determined from the following equation: ##EQU1## where V.sub.I equals the rate at which the chilled fluid is circulated through the catheter; .DELTA.T.sub.I equals the difference between the temperature of the chilled fluid input to the catheter and the temperature of the fluid returning from the heart; .DELTA.T.sub.B equals the difference between the temperature of the blood leaving the heart before the chilled fluid is circulated and the temperature of the blood leaving the heart after the chilled fluid is circulated (after the temperature stabilizes); and C is a constant dependent upon the blood and fluid properties. The patent also teaches that the fluid may instead be heated so that it transfers heat to the blood flowing through the heart rather than chilled to absorb heat.
U.S. Pat. No. 4,819,655 further teaches that the cardiac monitoring system induces temperature variations in the pulmonary artery that are related to the patient's respiratory cycle and are therefore periodic at the respiratory rate. Accordingly, Webler suggests that the signal indicative of T.sub.B ' (the temperature of the chilled blood exiting the heart) should be processed through a Fourier transform to yield a period and amplitude for the respiratory cycle, the period or multiples of it then being used as the interval over which to process the data to determine cardiac output.
Another problem recognized by Webler is the delay between the times at which circulation of the chilled fluid begins and the temperature of the blood in the pulmonary artery reaches equilibrium, which is caused by the volume of blood surrounding the catheter in the right ventricle and in other portions of the heart. The patent suggests introducing a generally corresponding delay between the time that temperature measurements are made of the blood before the chilled fluid is circulated and after, for example, by waiting for the .DELTA.T.sub.B value to exceed a level above that induced by respiratory variations. However, for a relatively large volume heart and/or very low cardiac output, the T.sub.B ' data do not reach equilibrium in any reasonable period of time. The quantity of blood flowing through the large volume heart represents too much mixing volume to accommodate the technique taught by Webler for processing the data to determine cardiac output. As a result, the measurement period for equilibrium must be excessively long to reach equilibrium, thereby introducing a potential error in the result due to either a shift in the baseline temperature of the blood or changes in the cardiac output. For this reason, the technique taught by Webler to determine cardiac output using the data developed by his system is not practical in the case of large blood volumes in the heart and/or low cardiac outputs.
The technique disclosed by Webler also assumes that all of the energy absorbed by a chilled fluid (or lost by a heated fluid) represents heat transferred between the fluid and the blood in the heart. This assumption ignores the heat transfer that occurs between the fluid and the mass of the catheter. A somewhat smaller source of error arises due to the energy required to change the temperature of the small thermal mass of the thermistor bead that monitors the temperature of blood leaving the heart. For long measurement periods, these errors can generally be ignored. In addition, if the thermistor bead is selected to have a very small mass and fast response time, its error contribution may be insignificant. However, as the measurement period becomes shorter, the effect of these error sources becomes increasingly more important.
Instead of cooling (or heating) the blood in the heart by heat transfer with a circulating fluid to determine cardiac output, the blood can be heated with an electrical resistance heater that is disposed on a catheter inserted into heart. The apparatus required for this type of injectateless cardiac output measurement is significantly less complex than that required for circulating a fluid through the catheter. An electrical current is applied to the resistor through leads in the catheter and adjusted to develop sufficient power dissipation to produce a desired temperature rise signal in the blood. However, care must be taken to avoid using a high power that might damage the blood by overheating it. An adequate signal-to-noise ratio is instead preferably obtained by applying the electrical current to the heater at a frequency corresponding to that of the minimum noise generated in the circulatory system, i.e., in the range of 0.02 through 0.15 Hz. U.S. Pat. No. 4,236,527 (Newbower et al.) describes such a system, and more importantly, describes a technique for processing the signals developed by the system to compensate for the above-noted effect of the mixing volume in the heart and cardiovascular system of a patient, even one with a relatively large heart. (Also see J. H. Philip, M. C. Long, M. D. Quinn, and R. S. Newbower. "Continuous Thermal Measurement of Cardiac Output," IEEE Transactions on Biomedical Engineering, Vol. BMI 31, No. 5, May 1984.)
Newbower et al. teaches modulating the thermal energy added to the blood at two frequencies, e.g., a fundamental frequency and its harmonic, or with a square wave signal. Preferably, the fundamental frequency equals that of the minimal noise in the cardiac system. The temperature of the blood exiting the heart is monitored, producing an output signal that is filtered at the fundamental frequency to yield conventional cardiac output information. The other modulation frequency is similarly monitored and filtered at the harmonic frequency, and is used to determine a second variable affecting the transfer function between the injection of energy into the blood and the temperature of the blood in the pulmonary artery. The amplitude data developed from the dual frequency measurements allows the absolute heart output to be determined, thereby accounting for the variability of fluid capacity or mixing volume.
Newbower et al. does not address correcting for errors due to the thermal mass of the catheter and the thermistor bead used to monitor the temperature of blood leaving the heart. Furthermore, the technique taught in Newbower et al. requires matching the dual frequency data to a predefined curve using a best fit algorithm, to determine the absolute cardiac output. Accordingly, the results are not as accurate as may be desired, particularly in the presence of noise.
It is preferably that a non-injectate method for determining cardiac output be based on measured output data processed using a technique that does not require fitting the output data to a curve. Cardiac output should also be determined by a method that compensates for the mixing volume of the heart, regardless of its relative size, and also compensates for the thermal mass of the catheter and the thermistor bead used to produce the output signal. The foregoing aspects and many of the attendant advantages of the present invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings.